RoboHelp 2017/2019 Classic: How can I generate PDF Output for merged projects?

Hi,

for our help system https://help.celum.com we generate the output from 4 different projects - a dedicated project for each of our product lines and a parent project which merges all of them together and contains some marketing intro topics.

If I try to generate the "Printed documentation" output for the merged project, it only includes the marketing topics that are actually part of the merged parent project. All the child project topics are not included.

Is there any way to generate a single PDF from a merged project that has 1 parent and multiple child projects? Or is it simply not possible? Do I have to generate a Word document of each child project and then merge them all together before converting to PDF manually?

We are using Adobe RoboHelp 2019 Classic (have not switched to the Reimagined UI yet). 

Any help would be greatly appreciated.

Cheers, Birgit